Abstract - This work presents an analysis based on experimental measurement of the influence of gas velocity, applied potential, diameter of the active electrode and distance between two consecutive active electrodes in the performance of a wire-plate electrostatic precipitator. The dimensions of the experimental equipment were as follows: length = 0.5 m, height = 0.3 m, width = 0.3 m, number of ducts = 3, distance between the collection electrodes = 0.1 m and collection area = 0.9 m2. The particulate material used in the tests was a phosphatic concentrate with a density of 3.0 x 103 kg/m3 and a mean particle size of 8.0 mm. The performance of the precipitator was evaluated by measuring its overall collection efficiency, obtained by isokinetic sampling at the entrance and exit of the experimental system. The results were compared to predictions obtained using the classic Deutsch-Anderson equation as well as the one from Zhao et al. (1996). A correction factor is proposed. Keywords: electrostatic precipitation, gas cleaning, particle collection.

INTRODUCTION

Although the behavior of electrostatic precipitators is relatively well studied, there is a lack of experimental data on their performance due to the high number of parameters, which include particle properties (density, electrical resistivity, size distribution, etc.), process variables (gas velocity, particle concentration, etc.), construction and geometrical parameters (electrode type and shape, space between the plates and electrodes, etc.) and operational variables (applied potential, cleaning frequency, etc.).

Particles contained in gaseous currents are collected by means of electrostatic precipitation after passing through a strong electric field, produced by high electrical potential with positive or negative polarity applied to a discharge electrodes system. When particles go through this electric field, they acquire electric charges and are attracted to collection electrodes. The particles are deposited on collection plates where they lose their charge, and the material is periodically removed by cleaning mechanisms as it accumulates. In spite of the apparent simplicity of the process, the large number of variables as well as the interactions between them make this a complex phenomenon.

In attempts to maximize the performance of the precipitators, many researchers (e.g., Chang and Bai, 1999; Miller et al., 1998; Navarrete et al., 1997; Acha et al., 1996; Abdel-Sattar, 1991) have addressed the problem of quantifying the influence of some of these variables. However, experimental data on the performance of electrostatic precipitators is still relatively scarce.

This work evaluates experimentally the influence of gas velocity, applied potential, diameter of the active electrodes and distance between electrodes in the performance of a plate-type precipitator. For this purpose, the applied potential was varied for two gas velocities, three active electrode diameters and two distances between electrodes.

Collection Efficiency

Evald Anderson's pioneering work in 1919 and Walther Deutsch's in 1922, gave origin to the classic equation of collection efficiency for electrostatic precipitators, known as the Deutsch-Anderson equation given by

It is worth noting that Eq. (1) was originally proposed for efficiency calculation, for which the overall efficiency for a given precipitator could be calculated as

Based on this early work, several researchers developed new equations for prediction of collection efficiency, and a recent paper by Zhao et al. (1996) gives a thorough review on the subject. Table 1, taken from their work, lists some of the correlations proposed during the past four decades. However, according to Zhao et al., although many of these correlations present satisfactory results, their use has been restricted by their complexity, by the limiting aspect of the assumptions on which they are based and also by the presence of empirical constants that are dependent on the operational conditions. Therefore, the Deutsch-Anderson equation is still widely used in the precipitators project, using an effective migration velocity, wef, obtained from experimental data, instead of wi. In this case, Eq. (1) is written as

For the empirical constant, k, the authors recommend the value of 1/7.

MATERIALS AND METHODS

Experimental Set-up

A sketch of the experimental module used in this work is shown in Figure 1. The electrostatic precipitator was a wire-plate system with three ducts, whose main dimensions are shown in Figure 1b. The precipitator was energized by a high-voltage DC supply (Spellman SL1200) to generate a positive corona. Values of the output voltage and current were read directly from the power supply.

The mass concentration of the particles before and after the precipitator was measured isokinetically by sampling the gas flowing through probes installed at the entrance and at the exit of the precipitator. The sampling system was composed of probes, filters, flow meter/controller and vacuum pump, as illustrated in Figure 2. The sampling velocity was kept the same as the duct flow velocity previously measured with a Pitot tube.

Material

The particulate material used in this work was a phosphatic concentrate, whose main characteristics are presented in Table 2.

The resistivity of the particulate material was obtained experimentally according to the methodology described by Coury (1983) and the density measured in an Accupic helium picnometer, from Micromeritics. Figure 3 shows the size distribution of dust particles measured in a Malvern Mastersizer.

Variables and Ranges

In order to verify the influence of applied potential (V), gas velocity (vo), diameter of the active electrodes (2rSE) and distance between two consecutive active electrodes (2c) on the performance of the precipitator, the following values were used: V = 17 to 27 kV; vo = 0.4 and 0.7 m/s; 2rSE = 0.2, 0.5 and 0.8 mm ; 2c = 0.1 and 0.2 m.

Experimental Procedure

The experimental overall collection efficiency, hT, was calculated according to the following definition:

Inlet particle concentration was kept constant in each test. From test to test, it was varied between 1.2 x 10-3 and 3.0 x 10-3 g/l, and its possible effect on precipitation performance was assumed to be negligible. The average values for relative humidity, pressure and temperature of the gas for all tests were

30%, 700 mmHg and 32 ° C, respectively.

The plates were cleaned only at the end of each test in order to avoid influencing the results by interruptions.

Experimental Design

The experimental tests were performed according to a mixed 22 x 32 factorial design to allow a more detailed analysis. Table 3 lists the levels used for each one of the variables, as well as their coded values. The coded variables were calculated as follows:

RESULTS AND DISCUSSION

The Effect of Gas Velocity and Applied Potential on Collection Efficiency

The influence of air velocity and applied potential on the performance of the precipitator is shown in Figure 4. It can be verified that the increase in gas velocity caused decrease in the overall collection efficiency, confirming the tendency predicted by the Deutsch-Anderson (Eq. 1a) and Zhao et al. (Eq. 3) equations. It can also be seen that, in most cases, collection efficiency increases as the applied potential is increased up to a maximum point, where it begins to decrease. This behavior is due to the onset of the back corona, which reduces the sparkover voltage and provokes the re-entrainment of particles into the gas flow.

Back corona, reverse corona or reverse ionization, are the names given to the localized discharges that occur on the surface of collection electrodes when they are covered with a layer of high resistivity particles. This phenomenon can occur both in positive as well as negative corona, although its effects are less accentuated in positive corona (White, 1963). Reverse corona is normally identified by the erratic behavior of the current in relation to time of operation. In practice, it can be easily seen by the occurrence of sparks during the test. Figure 5a compares the behavior of the current as a function of time of operation for two tests, with and without back corona. Figure 5b shows the behavior for two tests in which reverse corona has occurred. This phenomenon occurred for all active electrodes utilized, but its intensity varied with applied potential. Table 4 presents a summary of the main tests performed; those in which back corona was clearly observed are marked.

Analyzing Table 4 together with Figures 4a and 4b, it can be verified that reverse corona had a negative influence on the performance of the precipitator and that influence was more accentuated for the velocity of 0.7 m/s. It is interesting to note that back corona minimized the negative effect of the increase in velocity as can be seen in Figure 4c: the curves converge as the applied potential approaches 25 kV.

The Effect of Active Electrode Diameter on Collection Efficiency

To evaluate the effect of the diameter of the active electrode on the performance of the precipitator, only the tests where back corona was not detected were taken into account. Observing the behavior of the curves in Figure 6, a decrease in efficiency with the increase in active electrode diameter is verified. This becomes less accentuated as applied potential increases.

Electrode diameter also influenced the onset of back corona, as can be verified in Table 4: reverse corona appeared for smaller potentials when the diameter of the electrode was 0.2 mm for both velocities tested. Conversely, for the electrodes with higher diameters (0.5 and 0.8 mm), the phenomenon only appeared in the tests performed at 25 kV. In these cases, back corona was more intense for the diameter of 0.5 mm.

The Effect of the Distance Between Two Consecutive Active Electrodes (2c) on Overall Collection Efficiency

The distance between the active electrodes is a very important parameter in the electrostatic precipitators project. Miller et al. (1998) suggested that this distance should be as large as possible, so that the precipitator would work farther from the point of rupture of the gas dielectric constant. Nevertheless, they verified that the collection efficiency for fine particles is enhanced when 2c < s. Riehle (1997) comments that in most of the electrostatic precipitators projects the s/2c ratio is normally assumed to be equal to one. However, according to Riehle, this is not fortuitous because it is exactly in that configuration that the current density reaches its maximum value. In the present work, two distances (0.1 and 0.2 m) were tested and Figure 7 presents the results obtained.

It can be seen that the increase in 2c resulted in a decrease in collection efficiency. However, the decrease in electrode diameter minimized that effect. This result suggests the existence of an optimum relationship between the diameter of the electrodes and the distance between them. It is interesting to note that reverse corona did not occur in any of the tests carried out with 2c = 0.2 m. This explains the proximity of the curves for larger potentials, where this phenomenon occurred only for the tests with 2c = 0.1 m.

Comparison of the Theory Vs. Experimental Data for Overall Efficiency

Departing from the Deutsch-Anderson equation (Eq. 1a) and utilizing the experimental values of hT, it was possible to calculate the value of wef for each test performed. Observing these values, it was verified that wef is a function of variables V, 2c, 2rSE and vo. To adjust that function to the experimental data, the criterion of the mixed factorial design (22 x 32) was used. Utilizing the least squares method, the following expression for wef was adjusted for a correlation coefficient, R2, of 0.93:

where wef is given in m/s. The coded values x1 to x4 are listed in Table 3. This correlation suggests that there is no single value of wef that can cover the whole range of parameters studied here.

Figure 8 shows a comparison of the experimental results with the Deutsch-Anderson equation (Eq. 1a) with wef given by Eq. (6). As expected, there is a good fit of the experimental data with the Deutsch-Anderson equation adjusted by Eq. (6). In the 12 tests performed to test the correction proposed by Eq. (6), the maximum deviation was of 3.2%.

In Figure 9 the experimental data are compared with the correlation proposed by Zhao et al. (Eq. 3). The value for the constant, k, that was best adjusted to the data was 1/3 (the average value for all tests), which differs from the value suggested by the authors (1/7), but is within the recommended range (1/10 < k <1/2). One can observe that the correlation does not represent the experimental data well.

When analyzing the values for k calculated from the Zhao et al. equation (Eq. 3), it was verified that, like wef, k is a function of variables V, 2c, 2rSE and vo. To adjust that function to the experimental data, the factorial design described previously was again utilized. The expression obtained for k is given below for a correlation coefficient, R2, of 0.91:

where k is dimensionless.

Figure 10 shows the experimental results compared to Eq. (3) (Zhao et al., 1996) with the value of k given by Eq. (7). As can be seen, there is a good fit between the experimental data and the correlation. The maximum deviation in the 12 tests performed was of 2.2%.

CONCLUSIONS

An experimental study of the overall collection efficiency of a wire-plate electrostatic precipitator was carried out and the main conclusions were as follows:

1) gas velocity was shown to influence collection efficiency, and this effect is more accentuated for low potentials and in the presence of back corona;

2) the performance of the electrostatic precipitator was positively influenced by the increase in applied potential, but it was limited by the onset of reverse corona;

3) the increase in the diameter of the discharge electrode reduced collection efficiency for the same applied potential, but it improved operation at a higher potential by avoiding the onset of back corona;

4) for the electrode with the smallest diameter (0.2 mm), the variation in 2c from 0.1 m to 0.2 m did not influence the performance of the precipitator. That same variation in 2c for the other two diameters (0.5 and 0.8 mm) resulted in a decrease in collection efficiency;

5) the increase in the distance between the collection electrodes from 0.1 m to 0.2 m prevented the onset of reverse corona up to 25 kV for all tests performed, independent of gas velocity and electrode diameter;

7) the Deutsch-Anderson (Eq. 1a) and Zhao et al. (Eq.3) equations, as originally proposed, did not represent the experimental data well. Parameters wef and k were shown to be functions of V, 2rSE, vo and 2c;

8) empirical correlations were adjusted for estimating wef and k from the experimental values obtained according to a mixed factorial design (22 x 32) and are given by Eqs. 6 and 7 for the Deutsch-Anderson and the Zhao et al. equations, respectively. The fitted curves presented a maximum deviation of 3.5% throughout the range studied.

ACKNOWLEDGEMENTS

The authors are indebted to PRONEX/FINEP, FAPESP and CAPES for the financial support given for this work.

NOMECLATURE

A

total collection area, m2;

ce

inlet particle concentration;

cs

outlet particle concentration.

dm50

mass mean diameter of the particles, m;

E

electric field, V/m;

f(i)

mass fraction of particles in the ith size range, fed into the system.